How to use IndexOf() method of List<object>

Question

All the examples I see of using the IndexOf() method in List<T> are of basic string types. What I want to know is how to return the index of a list type that is an object, based on one of the object variables.

List<Employee> employeeList = new List<Employee>();
employeeList.Add(new Employee("First","Last",45.00));

I want to find the index where employeeList.LastName == "Something"

Chichihaerh

Is there a way to do this without lambda expressions? I'm stuck using .net 2.0 – Chichihaerh

Can you use my solution below? – Boxthorn

Answer 1

int index = employeeList.FindIndex(employee => employee.LastName.Equals(somename, StringComparison.Ordinal));

Edit: Without lambdas for C# 2.0 (the original doesn't use LINQ or any .NET 3+ features, just the lambda syntax in C# 3.0):

int index = employeeList.FindIndex(
    delegate(Employee employee)
    {
        return employee.LastName.Equals(somename, StringComparison.Ordinal);
    });

Gastrulation

I confirm this does the job. However the project doesn't use Linq. We are using .NET 2.0 – Chichihaerh

@Bay: you can use the old (pre-C# 3) syntax for the anonymous delegates, but the original I wrote would work find in .NET 2 as long as you're compiling with the C# 3 compiler (C# 3.0 and .NET 3.0 are not dependent on each other). – Gastrulation

Answer 2

public int FindIndex(Predicate<T> match);

Using lambdas:

employeeList.FindIndex(r => r.LastName.Equals("Something"));

Note:

// Returns:
//     The zero-based index of the first occurrence of an element
//     that matches the conditions defined by match, if found; 
//     otherwise, –1.

Big

Answer 3

you can do this through override Equals method

class Employee
    {
        string _name;
        string _last;
        double _val;
        public Employee(string name, string last, double  val)
        {
            _name = name;
            _last = last;
            _val = val;
        }
        public override bool Equals(object obj)
        {
            Employee e = obj as Employee;
            return e._name == _name;
        }
    }

Nicki

This is the one we used to do in the fisrt dotnet version – Nicki

This is actually the only answer that answers the original question - how to use IndexOf() with any object. Everyone else has just suggested using FindIndex() instead ... which is OK, but this answer should get more credit I think. – Bland

Answer 4

Sorry, one more for good measure :)

int index = employees.FindIndex(
      delegate(Employee employee)
        {
           return employee.LastName == "Something";
        });

Edit: - Full Example in .NET 2.0 Project.

class Program
{
    class Employee { public string LastName { get; set; } }
    static void Main(string[] args)
    {
        List<Employee> employeeList = new List<Employee>();
        employeeList.Add(new Employee(){LastName="Something"});
        employeeList.Add(new Employee(){LastName="Something Else"});
        int index = employeeList.FindIndex(delegate(Employee employee) 
                           { return employee.LastName.Equals("Something"); });
        Console.WriteLine("Index:{0}", index);
        Console.ReadKey();
    }
}

Boxthorn

Yes, this satisfies my actual request. I am leaving the check mark on the lambda version for other users as they might be using 3.5. Thanks. – Chichihaerh

Your example uses the initializer syntax that's only in C# 3. The OP hasn't clearly distinguished whether he's using C# 3 or C# 2, so it's hard to tell what he wants. – Gastrulation

Answer 5

The answer is for those coming here to know why IndexOf() doesn't work.

Your class must override Equals method of object possessing the following declaration.

public override bool Equals(object obj)

Peso

Answer 6

I prefer like this

    private List<Person> persons = List<Person>();

            public PersonService()
            {
                persons = new List<Person>() { 
                    new Person { Id = 1, DOB = DateTime.Today, FirstName = "Pawan", LastName = "Shakya" },
                    new Person { Id = 2, DOB = DateTime.Today, FirstName = "Bibek", LastName = "Pandey" },
                    new Person { Id = 3, DOB = DateTime.Today, FirstName = "Shrestha", LastName = "Prami" },
                    new Person { Id = 4, DOB = DateTime.Today, FirstName = "Monika", LastName = "Pandey" },
                };
            }

public PersonRepository.Interface.Person GetPerson(string lastName)
        {
            return persons[persons.FindIndex(p=>p.LastName.Equals(lastName, StringComparison.OrdinalIgnoreCase))];
        }

Malonis